Acoustic device



Dec. 27, 1938. A. E. SWICKARD ACOUSTIC DEVIG E Filed J1ily 28, 1936 mumman, M M M nzlmml-w NIT" 34 I/VVENTOR B A. E. SW/CKARD A TTOR/VEVPatented Dec. 27, 1938 ACOUSTIC DEVICE Andrew E. Swickard, East Orange,N. 3., assignor to Bell Telephone Laboratories, Incorporated, New York,N. Y., a corporation of New York Application July 28, 1936, Serial No.92,971

5 Claims.

This invention relates to acoustic devices, and, more particularly, to amicrophone or transmitter of the conductive ribbon diaphragm type.

An object of the invention is to improve the 5 structure and operatingcharacteristics of microphones.

A feature of the invention comprises providing a microphone with adiaphragm having plurality of portions, one of which is responsive tothe velocity component and another to the pressure component of incidentsound waves, one side of one portion being isolated from sound waves.

Another feature comprises isolating one side of a portion of thediaphragm by providing an enclosure defining an air chamberthereadjacent.

Still another feature comprises so shaping the enclosure that theeffects of diffraction about the enclosure are minimized.

A further feature comprises providing acoustic damping on the enclosedside of the diaphragm, preferably of such value that mechanicalimpedance of the diaphragm portion is negligible compared to theacoustic resistance of the damping over the entire frequency range, andthe acoustic impedance of the air chamber stiffness is comparably smallat the low freqencies.

Other and further features will be evident from the descriptive matterwhich follows.

A more complete understanding of this invention will be obtained fromthe detailed description which follows, taken in conjunction with theappended drawing, wherein:

Fig. l is a view in perspective of an acoustic device embodying theinvention;

Fig. 2 is a cross-sectional view of the device of Fig. 1 taken along theline 2-2 thereof;

Fig. 3 is a cross-sectional view, taken along the line 33 of Fig. 2 of afragment of the device of Fig. 1; and

Fig. 4 is a front elevational view of a housing and support for thedevice of Fig. 1.

The acoustic device of the drawing is a ribbon type microphone,designated generally as 20,

comprising a horseshoe or U-shaped permanent magnet 24, for example, ofcobalt steel, and a pair of pole-pieces 22, for example, of magneticiron, secured by flange portions 23 and screws 24 to the ends of themagnet, and having straight,

elongated, magnetic air-gap defining portions 25 having lateral surfaces23 converging to provide the air-gap 2'1. A conductive ribbon diaphragm28, preferably transversely corrugated, about .2 mil in thickness, andof a high-strength, lightweight material, such as aluminum alloy orduralumin, is positioned in the air-gap. The diaphragm is divided intotwo sections 29, 30 and is secured near its ends 3| and at itsmid-portion by clamping members 32 of insulating material. The section29 is exposed on its opposite 5 sides to the atmosphere and to soundwaves; it

is adapted and intended to respond to the velocity component of theincident sound waves. The section 38, however, is exposed on one sideonly to incident sound waves, its other side being 10 enclosed; it isadapted and intended to respond to the pressure component of the soundwaves. One side of section 30 is enclosed by a hollow container orenclosure 33, preferably streamlined and substantially oval-shaped. Thecontainer is 15 secured in place by the screws 34 and nuts 35, thescrews 34 and nuts 36 also clamping together the pole-pieces and theinsulating blocks. The container is in two portions 31, 38 to form thechamber 43, secured together by suitable 20 fastening means, such asscrews 39 and nuts 40, and has an elongated slot or opening 4| adjacentone or the rear side of the pressure component section of the diaphragm.An acoustic resistance 42, preferably silk fabric having sub- 25stantially the characteristic impedance of air, extends across thisopening, being held in position by the nuts 35.

If a combination comprising a pair of ribbon diaphragms, or a ribbondiaphragm having two 30 sections, one ribbon or section being responsiveto the velocity component and the other to the pressure component, is tooperate as a unidirectional microphone, the motion of the pressurecomponent ribbon or section should be in phase 35 with the sound fieldpressure. The ribbon or section is not responsive to the pressurecomponent unless one side is enclosed, as shown, so that no sound wavesare incident thereon. Damping, in the form of the silk fabric, must be40 introduced in series with the pressure component section to ensurethat the mechanical impedance of that ribbon section is negligiblecompared to the damping resistance, or the proper phase relationshipdoes not result. The volume of the 45 chamber provided by the containermust be so large that the acoustic impedance of the air chamberstiffness is small at the lower frequencies as compared to the acousticresistance of the damping material. In a device constructed in 50accordance with this invention, the chamber volume was about fifteencubic inches. The streamlining of the container minimizes the effect ofsound diffraction about the microphone.

The acoustic device 23 may be enclosed in a 55 perforated housing orcase Q5 supported at the upper end of a pedestal 4E5 rising from a flooror table base member 41.

While this invention has been disclosed with reference to a singleembodiment, it will be understood that it is not specific thereto, butis to be considered as limited only by the scope of the appended claims.

What is claimed is:

1. A microphone having a ribbon diaphragm, one section of which isresponsive to the velocity component of a sound Wave, means forrendering another section of the diaphragm responsive to the pressurecomponent of said wave, said means comprising an ovate hollow containerenclosing one side of said other diaphragm section and providing an airchamber thereadjacent, and a fabric acoustic screen disposed adjacentthe closed side of said other diaphragm section and completely acrossthe entrance to said container.

2. A microphone comprising pole-pieces spaced apart to define anelongated air-gap, a conductive ribbon diaphragm disposed in saidair-gap, means for securing said diaphragm at its ends and intermediateportion whereby the diaphragm is divided into two sections, one sectionbeing responsive to the velocity component of an incident sound Wave,ovate hollow means enclosing one side of the other section and having anopening theread-jacent, and a layer of acoustic resistance fabricdisposed entirely across said opening and spaced from said diaphragm.

3. A microphone comprising a conductive ribbon diaphragm, spacedpole-pieces defining an air-gap in which said diaphragm is positioned,

clamping blocks of insulating material on opposite sides and at each endof said pole-pieces and diaphragm, an enclosure for one side of saiddiaphragm having an opening adjacent the diaphragm and defining an airchamber on said one side, fabric acoustic resistance means disposedacross the entire opening and spaced from said diaphragm, and means forclamping together said pole-pieces, diaphragm, insulating: blocks andenclosure.

4. A microphone comprising a conductive ribbon diaphragm, pole-piecesdefining an air-gap for said diaphragm, means defining an air chamberenclosing one side of said diaphragm and having an opening adjacent saiddiaphragm, and fabric acoustic resistance means disposed entirely acrosssaid opening and spaced from said diaphragm.

5. A microphone comprising a conductive ribbon diaphragm, pole-piecesdefining an air-gap for said diaphragm, means defining an ovate airchamber enclosing oneside of said diaphragm and having an openingadjacent said diaphragm, and fabric acoustic resistance means disposedacross said opening and spaced from said diaphragm, the mechanicalimpedance of the diaphragm being negligible compared to the dampingresistance of said acoustic resistance means, and the air chamber volumebeing so large that the acoustic impedance of the air chamber stiifnessis small even at the low frequencies compared to the damping resistance.

ANDREW E. SWICKARD.

